Structural Controls of the San Emidio Geothermal System, Northwestern Nevada

Detailed geologic analyses integrated with geophysical and well data have provided insight into the kinematics, stress state, and structural controls of the San Emidio geothermal system. San Emidio lies within the Basin and Range province of northwestern Nevada, ~100 km north-northeast of Reno. The San Emidio area is dominated by middle to late Miocene Pyramid sequence volcanic rocks and late Miocene to recent sedimentary rocks, all overlying Mesozoic metasedimentary rocks. Currently, a small geothermal power plant produces 3.6 MWe from a 152°C reservoir at 520 m depth at the south end of the active San Emidio fault system. Abundant hydrothermal alteration along this fault zone, including acid-sulfate leaching and bleaching, native sulfur deposits, and boiling groundwater at depths less than 100 m, suggest, however, that the San Emidio geothermal resource extends several kilometers northward from the currently producing well field. Thermal anomalies identified along this fault zone also extend northward from the current production zone and intersect alteration around the relatively young (Pleistocene/Pliocene) Wind Mountain epithermal mineral deposit. Structural and lithologic similarities are established between this epithermal deposit and the modern geothermal system. Kinematic analysis, including slip and dilation tendency analysis, suggest that north-northeast striking faults and fractures are favorably oriented for fluid flow under west-northwest-directed extension. The San Emidio geothermal system and Wind Mountain epithermal deposit occupy intersections between favorably oriented north-northeast-striking normal faults and multiple closely-spaced north-striking normal faults. Additionally, these intersections correspond to right steps in hard-linked, north-striking, normal fault zones along the west flank of the northern Lake Range. Favorably oriented faults and fractures accompanied by increased fault and fracture density at fault intersections likely produce the permeability necessary for deep fluid circulation within the San Emidio system. Preliminary analysis of similar right-steps in mainly north-striking normal fault systems elsewhere within the Basin and Range province suggests that structural characteristics may serve as a viable and complementary exploration tool for discovering both epithermal mineral deposits and geothermal systems in extensional tectonic settings